[en] Fission times of lead and uranium nuclei have been measured at GANIL by the crystal blocking method. The inverse kinematics was used. Fragment atomic numbers and total excitation energies were determined. For data analysis, full Monte-Carlo trajectory calculations were used to simulate the blocking patterns. The effect of post-scission emissions, included in our simulations, is discussed. At high excitation energies, the scissions occur dominantly at times shorter than 10^-19 s, whereas at low excitation energies (E^* < 250 - 300 MeV), scissions occurring at much longer times with sizeable probabilities are observed both for uranium and lead nuclei, leading to average scission times much longer than those inferred from pre-scission emission. (authors)